Clean, efficient and versatile, H2—O2 (air) polymer electrolyte membrane fuel cells (PEMFCs) are seen as worthy alternatives to a wide range of conventional power generation devices such as internal combustion engines, batteries and diesel-fuelled back-up power systems. PEMFCs generate electricity via two electrochemical reactions that involve the oxidation of hydrogen at the anode (2H2→4H++4e−) and the reduction of oxygen at the cathode (O2+4H++4e−→2 H2O), thus producing only water and heat. Due to the rather low operating temperature of PEMFCs (ca. 80° C.), catalysts play an essential role in boosting the reaction kinetics to produce the desired high power densities.
Today, the only viable electrocatalysts used in PEMFCs are platinum-based. Platinum is considered to be a “noble” metal, such as gold, for example. In PEMFCs, 90% of the platinum is needed at the cathode due to the sluggishness of the oxygen reduction reaction (ORR) compared to the fast hydrogen oxidation reaction at the anode. Despite improved platinum performances, the increasingly prohibitive cost of platinum remains a major obstacle for the commercial viability of PEMFCs.
In addition, the production of platinum in the natural world is rather limited. On the other hand, there are estimations that the demand for platinum as electrode catalyst will increase significantly as the demand for electric cars with fuel cells increases. Therefore, there is a fear of a still further rise in platinum prices. Accordingly, electrode catalysts which can be formed without using noble metals, such as platinum, are desirable.
Research activity into non-noble or non-precious metal catalysts (NPMC) for the ORR has grown considerably since 1964 when Jasinski observed that cobalt phthalocyanine catalyzed the ORR (REFERENCE 1). Such catalysts were first obtained by adsorbing Fe—N4 or Co—N4 macrocycles on a carbon support and pyrolysing the resulting material in an inert atmosphere (REFERENCE 2). Since, NPMC research using metal-N4 macrocycles has continued (REFERENCE 3-5).
A breakthrough was then achieved when it was revealed that these often-expensive macrocycles could, instead, be substituted by individual N and Co precursors (REFERENCE 6). This approach was followed by several groups (REFERENCE 2, 7, 8-17).
One approach in the synthesis of NPMCs for ORR has been to use NH3 as a nitrogen precursor. The catalysts are obtained by wet impregnation of a carbon black with an iron precursor like ironII acetate (FeAc), followed by a heat treatment, i.e. pyrolysis, in NH3. Herein, such electrocatalysts will be referred to as Fe/N/C catalysts. During pyrolysis at temperatures ≧800° C., NH3 partly gasifies the carbon support, resulting in a mass loss that depends on the duration of the heat treatment (REFERENCE 18). The disordered domains of the carbon support are preferentially gasified (REFERENCE 19-21). As a result, micropores are created in the carbon black particles. The mass loss (30-50 wt %) at which maximum activity is reached corresponds to the largest microporous surface area of the etched carbon, suggesting that these micropores (size ≦2 nm) host the catalytic sites (REFERENCE 19). In addition, the reaction of NH3 with the disordered carbon domains also produces the N-bearing functionalities needed to bind iron cations to the carbon support (REFERENCE 22-23).
Hence it has been proposed that each Fe/N/C catalytic site comprises an iron cation coordinated by four pyridinic functionalities attached to the edges of two graphene planes, each belonging to adjacent crystallites on either side of a slit pore in the carbon support (REFERENCE 19, 23). Thus, four factors have been identified as requirements for producing active Fe-based catalysts for ORR: (i) disordered carbon content in the catalyst precursor (REFERENCE 18); (ii) iron; (iii) surface nitrogen and (iv) micropores in the catalyst. Disordered carbon allows for the formation of micropores and nitrogen enrichment during pyrolysis in NH3, Fe and N are essential because they form an integral part of the catalytic site (REFERENCE 2), while micropores are required to host the catalytic site (REFERENCE 19-21).
For NPMCs, it is meaningful to speak in terms of volumetric activity for ORR. Conversion from A·g−1NPMC to A·cm−3electrode is described below. The volumetric activity is the product of the catalytic site density and the activity of a single site. The latter varies with voltage and is an intrinsic property of the catalytic site. Therefore, if the site is unchanged, increased volumetric activity can only be achieved by increasing the site density.
The volumetric catalytic activity of a catalyst may be marginally improved by increasing the Fe content. However, the inventors previously found increased activity only up to a presence of ca. 0.2 wt % Fe, beyond which the activity levels off and eventually decreases (REFERENCE 24). Therefore, a nominal Fe concentration of 0.2 wt % was at that time chosen for impregnation onto pristine non-porous carbon blacks.
Furthermore, when catalysts were prepared using the impregnation method on non-porous carbon black and pyrolysed in pure NH3, the micropore surface area of the resulting catalysts was shown to govern the catalytic activity because the nitrogen and iron content were usually non-limiting (REFERENCE 19).
FeAc was therefore impregnated onto highly microporous carbon supports followed by pyrolysis in NH3. Surprisingly, this did not improve the activity as compared to catalysts made with non-porous carbon supports. Instead, it was concluded that only the micropores created during pyrolysis in NH3 may host catalytic sites (REFERENCE 25). The inventors thus found that the micropores in the as-received microporous carbon blacks do not bear the surface nitrogen necessary to form catalytic sites. Furthermore, since these carbon blacks have little disordered carbon content, surface nitrogen is difficult to add during pyrolysis in NH3.
REFERENCE 26 gives conditions for measuring the volumetric activity of NPMCs in fuel cells.
REFERENCES 18 and 22 disclose the use of different carbon blacks and activated carbon in catalysts of the prior art.
REFERENCES 27 and 28 disclose the use of a carbon pretreated to add nitrogen and/or carbon with a nitrogen-containing molecule with iron acetate in catalysts of the prior art.
REFERENCE 29 discloses the use of iron, cobalt, chromium and manganese in catalysts of the prior art.
REFERENCES 30-38, 9, 39-46, discloses the use of at least the following non-noble metal precursors in catalysts of the prior art: cobalt porphyrin (Co tetramethoxyphenylporphyrin (TMPP)); iron acetate, Fe tetramethoxyphenylporphyrin (TMPP) on pyrolysed perylene-tetracarboxylic-dianhydride (PTCDA); Fe phthalocyanines; Fe and Co tetraphenylporphyrin; Co phthalocyanines; Mo tetraphenylporphyrin; metal/poly-o-phenylenediamine on carbon black; metal porphyrin; molybdenum nitride; cobalt ethylene diamine; hexacyanometallates; pyrrol, polyacrylonitrile and cobalt; cobalt tetraazaannulene; and cobalt organic complexes.
REFERENCES 29, 56 and 57 report the successful use of FeII acetate, cobalt acetate, copper acetate, chromium acetate, manganese acetate, nickel acetate, and ferrocene in prior art catalysts.
The present inventors and other authors have successfully used FeII acetylacetonate, FeII sulfate, FeIII chloride, FeIII nitrate, FeII oxalate, FeIII citrate, ChloroFe tetramethoxyphenylporphyrine, cobalt phthalocyanine, iron phthalocyanine, cobalt tetra-aza-annulene in catalysts of the prior art.
The present inventors have successfully used all the compounds between parenthesis and at least one compound of each family bellow in non-noble catalysts of the prior art: Phenanthroline (1,10-phenanthroline, Bathophenanthroline disulfonic acid disodium salt hydrate, 4,7-Diphenyl and 5,6-dimethyl phenanthroline, 4-aminophenanthroline); Phthalocyanine; Porphyrine; Phthalonitrile (4-Amino-phthalonitrile); Melamine; Hexaazatriphenylene; Tetracarbonitrile; Benzene-1,2,4,5-tetracarbonitrile; amino-acids; Polypyrrole; Polyaniline, Bismark Brown; and Bathocuproine (2,9Dimethyl-4,7-diphenyl-1,10-phenantroline). These results were unpublished before the present and were thus not part of the prior art available to the skilled person.
There remains a need for improved NPMCs to replace the Pt-based electrocatalysts used in PEMFCs.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.